This invention relates generally to golf clubs and, more particularly, to a hybrid shaft for improving the performance of golf clubs.
A modern golf club typically comprises a shaft, a head connected to the shaft""s tip end, and a grip disposed at the shaft""s butt end. Perhaps more than any other component, the shaft affects overall club performance. It is generally accepted that the optimal golf club shaft should have lightweight, high torsional stiffness, configurable bending stiffness, and provide moderately high swing weight and vibration-damping property.
A lightweight club generates greater acceleration, which in turn yields a higher swing velocity, than a heavy club does for the same amount of applied force. For clubs of similar weight and mass distribution, the greater the swing velocity, the farther the ball will travel when driven by the clubs. Torsional stiffness is preferred to limit unwanted angular deflection of the head about the shaft. This allows the face of the club head to impact the ball squarely so that the ball""s flight will follow a straight path. The torsional stiffness may be enhanced by enlarging the shaft""s diameter to increase polar moment of inertia, as well as by using materials having high Young""s modulus such as steel.
Skilled golfers who generate high swing velocity prefer clubs having a high bending stiffness. Average golfers, on the other hand, like clubs with low bending stiffness to take advantage of the xe2x80x9ckickxe2x80x9d resulting from shaft flexing during early part of swing and subsequent release as the golf club head impacts the ball. But golfers of all levels want a set of clubs having essentially the same swing weights to achieve consistent play. Swing weight is a measure of how the mass is distributed on a club and equates to the dynamic characteristics or xe2x80x9cfeelxe2x80x9d of the club. Tip-weighted shafts and/or heavy club heads tend to increase the clubs"" swing weights, while butt-weighted shafts and/or light club heads tend to decrease the clubs"" swing weights. A desirable club should also incorporate vibration-damping materials to absorb tactile shock and reduce acoustic propagation caused by the head striking the ball and/or ground.
There are essentially three existing club shaft designs, including metal shafts, composite shafts, and hybrid shaft of metal and composite material. Conventional shafts often optimize some of the characteristics mentioned above while compromising others.
The metal shaft, typically formed from steel, has long been the mainstay of golf club design. Steel has a high shear modulus, giving the shafts an inherently high torsional stiffness. Shafts of various bending stiffness and swing weights can be obtained by manipulating the thickness and lengths of the flexible tip portion and the rigid butt portion. Steel is also durable, strong, inexpensive, and provides great consistency of characteristics from one shaft to another. Unfortunately, steel is dense, and clubs having steel shafts are heavy, have relatively poor acceleration and consequently a lower swing velocity. Additionally, The conventional heavy rubber grip used with the steel shaft, comprising about 15% or more of the total mass of a typical driver or any fairway woods, further compounds the weight and weight distribution problems. Steel shafts are also very poor in absorbing shocks or damping vibrations.
Club shafts comprising composite materials such as graphite are commonly preferred over steel shafts because they can be made extremely lightweight and conform to desired flexural characteristics. The light graphite shaft affords the club with high swing velocity, which produces long drives. Primary drawbacks of the composite graphite designs are their high bending stiffness and low torsional stiffness. To provide a composite shaft with the same torsional stiffness as a metal shaft, particularly in the tip end where the torsional stress is great, many plies of high modulus fibers oriented at xc2x145 degree angle to the longitudinal axis of the shaft must be incorporated. Unfortunately, these fibers add significant bulk and weight in a particularly undesirable location on the shaft. Additionally, graphite composite shafts are more likely to break, particularly at the tip portion, the part of the shaft with the smallest diameter. Nonetheless, most golfers prefer composite shafts because they are lightweight and have a more pleasant xe2x80x9cfeelxe2x80x9d at impact than steel shafts. Composite shafts are also less sensitive to resonance phenomena since graphite composites are good vibration damping materials.
Hybrid shaft designs typically incorporate both metal and composite materials. U.S. Pat. Nos. 4,836,545 and 5,253,867 both disclose two-piece hybrid shafts that join together a lower metal tip portion with an upper composite butt portion. U.S. Pat. No. 5,028,464 discloses a golf club shaft having a laminated composite tube on the inside, a resin coat on the outside, and a transparent metallic layer disposed between the laminated tube and the resin coat. The transparent metallic layer is formed by depositing or plating a very thin layer of a metallic element onto a transparent cloth of organic and/or organic fibers impregnated with a thermosetting or thermoplastic resin. U.S. Pat. No. 5,083,780 discloses a tubular metal shaft having a short shell of reinforced composite molded over a predetermined location on the metal shaft to control the bending point of the shaft. U.S. Pat. No. 5,259,614 discloses a golf club shaft having a hollow steel tubular core and a composite filament spirally wound about the core to form a seamless jacket thereabout. U.S. Pat. No. 5,607,364 discloses a golf club shaft including a damping layer coated to the inner diameter of the shaft, and the damping layer is formed from a viscoelastic material. U.S. Pat. No. 5,904,628 discloses, among others, a lightweight hollow metal golf club shaft with an inflatable and flexible bladder which is pressurized by a gas to rigidify, reinforce and enhance the performance of the shaft. U.S. Pat. No. 6,139,444 discloses a hollow composite shaft having a preformed sheath metal tube surround the tip portion of the composite shaft as an external stiffener. U.S. Pat. No. 6,302,806 discloses a composite shaft having steel filaments aligned longitudinally in the tip portion for weighting, and steel filaments aligned longitudinally in the butt portion for reinforcement, thereby adjusting center of gravity and bending point of the shaft. U.S. patent application Ser. No. 09/248,569 discloses a hybrid shaft having a steel tip portion and a composite butt portion joined together, and the steel tip has a vibration damping member embedded therein. U.S. patent application Ser. No. 09/813,608 discloses a steel golf shaft having a steel tip portion and a steel butt portion joined by a composite pivot portion via connectors of various configurations.
There remains a need, however, for an improved golf club shaft that is light weight and provides the improved feel and vibration damping of fiber/resin composite shafts, as well as increased torsional stiffness and resistance to breakage of metal shafts.
The present invention is directed to a hollow golf club shaft of circular cross section having a tubular cover layer and a tubular core layer. The cover layer and the core layer are conjoined and coextend substantially the entire length of the shaft. Preferably, the cover layer has a thickness of less than about 0.2 inches, and the core layer has a thickness of less than about 0.3 inches. The cover layer is formed from an isotropic material such as metal matrix composites, metals and alloys thereof, including titanium, steel, stainless steel, aluminum, tungsten, nickel, copper, zinc, brass, bronze, magnesium, tin, gold, or silver. Preferably, the cover layer is a solid, continuous and non-porous metallic sheath.
The core layer is formed from a non-isotropic material, such as a reinforcement material, preferably impregnated with a vibration damping thermosetting or thermoplastic resin. Suitable reinforcement material include carbon fibers, graphite fibers, glass fibers, quartz fibers, boron fibers, ceramic fibers, ceramic whiskers, metal-coated fibers, ceramic-coated fibers, diamond-coated fibers, carbon nanotubes, extended-chain polyethylenes, poly-p-phenylenebenzobisoxazole fibers, metal fibers, polythenes, polyarylates, polyacetals, liquid crystalline polymers, aromatic polyesters, or polyallylates. Suitable resins include epoxy, polyester, polystyrene, polyurethane, polyurea, polycarbonate, polyamide, polyimide, polyethylene, polypropylene, polyether, polyvinyl halide, polyvinylidene halide, nylon, nylon 6, polyphenylene sulfide, polyether ether ketone, polyether ketone ketone, polyamide imide, polyether imide, polyaryl sulfone, polyether sulfone, or liquid crystal polymer. Preferably, substantially all of the reinforcement material is aligned longitudinally along the shaft. The thermosetting or thermoplastic resin has a loss factor of between about 0.2 and about 1.2 at 100 Hz and 68xc2x0 F., a shear storage modulus of at least about 1,000 psi, and a Young""s modulus of at least about 0.01 Mpsi. In one embodiment, the non-isotropic material is epoxy-impregnated carbon or graphite fibers, and the carbon or graphite fibers are present in an amount of from about 10% to about 80% by volume of the non-isotropic material.
Preferably, the shaft has a total weight of less than about 130 grams, the cover layer has a weight of less than about 80 grams, and the core layer has a weight of less than about 100 grams. The shaft may further have a vibration damping layer formed of at least one viscoelastic material disposed between the core layer and the cover layer, wherein the vibration damping layer comprises. Preferably, the viscoelastic material has a Young""s modulus of at least about 15 psi, a shear storage modulus of at least about 10 psi, a strain energy ratio of at least about 2%, and a loss factor of at least about 0.01 at a temperature range of xe2x88x9240xc2x0 C. to 100xc2x0 C. and a frequency range of 1 Hz to 10,000 Hz. More preferably, the vibration damping thermosetting or thermoplastic resin in the core layer is substantially the same as the viscoelastic material. The vibration damping layer preferably has a thickness of less than about 0.1 inches, covers at least about 5% of the shaft length, and can be a continuous layer, a discontinuous layer, a layer of uniformed thickness, a layer of non-uniformed thickness, a lattice network layer, a wound layer, a woven layer, a braided layer, or a laminar layer. The shaft may further comprises a reinforcing layer embedded in or disposed on an inner surface of the core layer, having a length of at least about 5% of the shaft length.
The invention is also directed to a hollow golf club shaft of circular cross section comprised of a tubular cover layer formed from an isotropic material having a Young""s modulus of at least about 5 Mpsi, and a tubular core layer formed from a non-isotropic material. Preferably, the cover layer and the core layer are conjoined and coextend substantially the entire length of the shaft. The non-isotropic material of the core layer includes a reinforcement material and a thermosetting or thermoplastic resin. The resin preferably has a Young""s modulus less than the isotropic material of the cover layer by at least about one order of magnitude. The cover layer has a thickness ranging from about 0.001 inches to about 0.15 inches, and the core layer has a thickness ranging from about 0.001 inches to about 0.2 inches. The shaft may further include a vibration damping layer between the cover layer and the core layer having a thickness ranging from about 0.0005 inches to about 0.05 inches, and/or a reinforcing layer embedded in or disposed on an inner surface of the core layer having a thickness of less than about 0.1 inches.
The invention is further directed to a hollow golf club shaft of circular cross section having a tubular cover layer formed from an isotropic material and a tubular core layer formed from a non-isotropic material, wherein the cover layer and the core layer are conjoined and coextend substantially the entire length of the shaft, and a volume ratio of the core layer to the cover layer is less than about 20:1. Any one location along the shaft preferably has a thickness ratio of the cover layer to the core layer ranging from about 5:95 to about 90:10. The outer surface of the core layer can be conjoined to the inner surface of the cover layer through structural adhesives, contact adhesion, physical bonding, or chemical bonding. Preferably, the structural adhesives is one-part heat-cured epoxies, two-part reaction-cured epoxies, two-part reaction-cured polyurethanes, acrylics, double-coated bonding tapes, pressure-sensitive adhesives, or heat-cured epoxy/acrylic hybrid adhesives.